I am using a bar graph, and the default tracker contains the label of each box followed by its value. Another possible option for me is to change the tracker message to have only the box value, without its name. How can it be done?

I don't think it is possible to hide the tracker. This is related to issue http://oxyplot.codeplex.com/workitem/9625 - it should be possible to change the mouse gesture to activate the tracker
- and to disable it. I have not implemented this yet, but hope to do it later this year.

To change the tracker text for the BarSeries/ColumnSeries, try to set TrackerFormatString = "{2}"